Here is a theory on how it works.
The difference is actually paid by the vendor. So why would someone
do that. The reason is, if they don't do that they won't have your
business. You would shop at the other place which advertised for a
lower price. The theory is, if the lower price is legitimate, they
would make some money too just like their competitor do. Amex is
happy to do that since if they don't offer this feature, you may be
using Master card or Visa instead.
http://www.800.com has a promotion of $75 off and free shipping.
They are the authorized dealers for Canon and I have brought mine
from them and I had price matched using the Amex. I got $250 check
from Amex. The equation is $900 - 75 - 250 = $575 and the peace of
mind with free shipping and no taxes. Is that not cool!
Hope this info is of some help!
Vasu.